Excessive moisture causes damage within many electronic enclosures such as lighting assemblies or electronic control units. For instance, electrical and electronic devices may be ruined due to excessive moisture within an enclosure. Examples of lighting assemblies which are subject to undesirable moisture include, for example, automotive headlamp units and other outdoor lighting assemblies where on/off cycling of a lamp within the enclosure results in moisture build-up. As used herein, the term “moisture” is intended to refer to water that is diffused or condensed, whether in liquid form or vapor form, from the ambient atmosphere.
The problem of moisture build up within enclosures is particularly acute in enclosures having polymeric components. For example, modern vehicle head lamps, brake lamps, running lamps, turn signal lamps, fog lamps, back-up lamps and parking lamps (collectively “lamps” or “vehicle lamps”) typically have one or more light bulbs located in an enclosed polymeric housing. It is critical to the effective operation of the lamp to prevent water, dirt, oils and the like from reaching the bulbs, the reflective surfaces, the lens or the housing. However, thermal cycling due to bulb operation, changes in the environment, and vehicle operation can cause moisture to condense on the interior of the housing or lens and inhibit light output from the lamp. Components within a lamp may be damaged by such condensation. Similar problems may arise in other electronic enclosures, such as electronic control units. Moisture buildup or condensation can contribute to corrosion, short circuits and the like.
The air outside of a lamp housing may be below the water vapor saturation point, and ambient air directed to flow through the housing may therefore have the capacity to remove condensation from the lamp housing by removing water vapor from the housing. Accordingly, one known means of reducing moisture related problems in automotive lamps is to use a vent to disperse moisture by providing greater airflow across or through the enclosure. Such vent systems attempt to reduce condensation by employing some means of increasing airflow through the lamp housing. However, it can be very difficult to provide sufficient airflow to reduce moisture condensation, because increasing the vent opening sizes can exacerbate problems such as contamination, etc. Many vent systems attempt to increase airflow by having vent openings in more than one location. The openings must often be placed in specific locations where airflow past the vent opening enhances airflow through the housing. Location of these vent systems may have a negative effect on other aspects of lamp performance.
Another means of reducing moisture in an enclosure is to place a drying agent or desiccant within the enclosure. Desiccants can operate by several fundamental mechanisms including absorption, adsorption, and reaction. Absorption occurs when a substance (e.g., water vapor) penetrates the inner structure of another (the absorbent). Adsorption occurs when a substance (e.g., water vapor) is attracted and held onto the surface of another (the adsorbent). Reaction occurs when the substance (e.g., water vapor) reacts with the desiccant to form a chemical bond with water. As the terms “desiccants” or “drying agents” are used herein, they are intended to refer to any material which absorbs, adsorbs, or reacts with water vapor from the air and is thereby able to reduce the moisture in the air within a lighting enclosure.
Many desiccants will desorb or release adsorbed or absorbed moisture when heated in a process called regeneration. Such desiccants are commonly referred to as regenerating desiccants. In contrast, non-regenerating desiccants retain adsorbed, absorbed, or reacted moisture when heated.
Moisture permeation through polymeric components of electronic enclosures contributes significantly to condensation problems in such enclosures. For example, many lighting enclosures are constructed of two polymer components. A opaque portion may be polypropylene and make up a significant portion of the total enclosure area. Lens components, which may be constructed of clear or substantially clear polycarbonate, have relatively high moisture permeation coefficients.